Extract main function into package

[fabxc]

If we extract the main logic, we can actually call it like a standard method. We can then actually make our functional test start Alertmanagers within the test.

This way we don't rely on an existing Alertmanager binary, which also has to be rebuilt before running functional tests. It will also reduce the overhead of our functional to make them less flaky.

[TheMeier]

@ultrotter do you have any opinion on this proposal